H1N1 Seroconversion Rates Varied in Singapore Cohorts

After 2009 wave, the rate was highest in military members, lowest in nursing-home residents, staff

By Rick Ansorge

HealthDay Reporter

TUESDAY, April 13 (HealthDay News) -- After the June to September 2009 wave of 2009 influenza A(H1N1) in Singapore, serologically determined infection rates varied widely by cohorts and age groups, suggesting that the context-specific risks of infection need to be considered, and interventions need to be tailored to the at-risk population, according to a study published in the April 14 issue of the Journal of the American Medical Association.

Mark I.C. Chen, Ph.D., of Tan Tock Seng Hospital in Singapore, and colleagues obtained serial serological samples between June 22, 2009, and Oct. 15, 2009, from four cohorts: 838 members of the general population, 1,213 military personnel, 558 staff from an acute care hospital, and 300 staff and residents from long-term care facilities.

In subjects with one or more follow-up serum samples, the researchers found that 312 military personnel (29.4 percent) seroconverted, compared with 98 community members (13.5 percent), 35 hospital staff (6.5 percent), and only three long-term care participants (1.2 percent). They also found an increased frequency of seroconversion among community participants from households in which one other member seroconverted (adjusted odds ratio [aOR], 3.32), and that older age was associated with decreased odds of seroconversion (aOR, 0.77 per 10 years).

"Although it appears that a large proportion of the Singapore adult population remain susceptible to the 2009 influenza A(H1N1) virus after the first epidemic wave, for a significant second wave to occur, a sufficient number of susceptible children may also be required for efficient transmission," the authors conclude. "These and other factors will need to be considered in the determination of optimal pandemic vaccination strategies for influenza A(H1N1)."